This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-061440, filed Mar. 9, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a frequency converter for use mainly in radio-communication equipment, and more particularly to a frequency converter for outputting a wide band signal.
In the receiver of radio-communication equipment, such as a portable radio-communication device, a frequency converter is arranged to convert a received signal to a signal having a predetermined frequency. As a frequency converter for treating a relatively narrow band signal, a well known one is disclosed, for example, in xe2x80x9cA Class AB Monolithic Mixer for 900-MHz Applicationsxe2x80x9d; Ken Leong Fong, Chistopher Dennis Hull, and Robert G. Meyer; IEEE J. Solid-State Circuits, vol. 32, No. 8, AUGUST 1997, p. 1166, which will be referred to as Publication 1.
FIG. 23 shows a frequency converter disclosed in Publication 1. The circuit includes a multiplying circuit formed of transistors Q101, Q102, and Q103. From the collectors of the transistors Q102 and Q103, an IF (intermediate frequency) signal is outputted as a current signal, which is a difference frequency signal between an RF (radio frequency) signal frequency and an LO (local oscillation) signal frequency. The output terminals of the multiplying circuit are connected to a load circuit including load resistors R100, R101, and R102, to which two LC parallel resonance circuits consisting of inductors L101 and L102 and capacitors C101 and C102 are connected, so that this part functions as a band pass filter. The load circuit is generally designed to match with the impedance of transmission lines.
In recent years, in place of narrow band modulation systems of several hundred kHz used for PHS (Personal Handy-phone System) and GSM (Global System for Mobile Communication), wide band modulation systems of several MHz or more, such as CDMA (Code Division Multiple Access) and OFDM (Orthogonal Frequency Division Multiplex) have come into use. When such a modulation signal with a wide band is converted into an IF signal with a frequency of about 200 MHz, the fractional band width of the signal becomes larger than that in the case of narrow band modulation systems by one multiple of ten.
The frequency converter shown in FIG. 23 has been developed for use in converting the frequency of a signal produced by narrow band modulation systems. Where the circuit is used for treating a signal with a wide fractional band width produced by wide band modulation systems, impedance matching can not be achieved over the entire signal band, thereby bringing about a fluctuation in the output signal level. If the LC parallel resonance circuits are provided with exterior elements to form a load circuit with a high Q (Quality) factor and thereby to expand the signal band, the resultant circuit structure becomes complicated. In this case, the number of exterior elements increases, thereby making integration circuits less compact and expensive, which is opposite to the desired requirements. In order to achieve impedance matching over a wide band, there is another method in which the Q factor of the resonance circuits is reduced. However, in this case, the output signal revel is lowered, thereby deteriorating the S/N ratio.
On the other hand, the structure shown in FIG. 24 is known as a frequency converter which can achieve impedance matching over a wide band. This circuit also includes a multiplying circuit formed of transistors Q101, Q102, and Q103. From the collectors of the transistors Q102 and Q103, an IF signal is outputted through an emitter follower circuit, which is formed of transistors Q104 and Q105 with current supplies CS104 and CS105 used as a load. This frequency converter can achieve impedance matching over a wide frequency range. The conversion gain is decided on the basis of the trans-conductance due to the transistors Q101, Q102, and Q103, and load resistors R101 and R102. The conversion gain is easily larger, and thus a sufficient output signal level is ensured.
However, the frequency converter shown in FIG. 24 has a problem in that an LO signal frequency component of a high level and its higher harmonic frequency components are included as undesired signal components, beside the desired IF signal component, in the output signal from the collector of the transistors Q102 and Q103. Such an undesired signal component of a high level causes transistors Q104 and Q105 in an output buffer circuit of the next stage to be saturated, thereby distorting the desired signals.
A method of utilizing a double balancing mixer is known to cancel the LO signal frequency component. However, in the double balancing mixer, the second harmonic of the LO signal frequency undesirably appears as an electric current flowing through the load resistor of the mixer. Particularly, where the LO signal frequency is high, the second harmonic component becomes large, thereby causing a problem as in the circuit shown in FIG. 24.
As described above, the conventional frequency converters have a problem in that the S/N ratio and the distortion characteristic have to be sacrificed where the circuits are designed to convert the frequency of a signal with a wide band.
An object of the present invention is to provide a frequency converter which can treat a wider band while maintaining a high S/N ratio and a low distortion.
According to a first aspect of the present invention, there is provided a frequency converter comprising:
a multiplying circuit configured to multiply an RF (Radio Frequency) signal and an LO (Local Oscillation) signal and output a difference frequency signal having a difference frequency between those of the RF and LO signals from an output terminal;
a load circuit connected to the output terminal of the multiplying circuit;
an output buffer circuit having an input terminal connected to the output terminal of the multiplying circuit and an output terminal for outputting a signal into a next stage; and
a notch circuit connected to the input terminal of the output buffer circuit and configured to have an impedance characteristic in which impedance is abruptly reduced to provide a valley point at an undesired signal frequency, in order to remove a component with the undesired signal frequency from the difference frequency signal.
According to a second aspect of the present invention, there is provided a frequency converter comprising:
a multiplying circuit configured to multiply an RF (Radio Frequency) signal and an LO (Local Oscillation) signal and output a difference frequency signal having a difference frequency between those of the RF and LO signals, the difference frequency signal comprising differential signal components outputted from first and second output terminals, respectively;
a load circuit connected to the first and second output terminals of the multiplying circuit;
an output buffer circuit having first and second input terminals connected to the first and second output terminals of the multiplying circuit, respectively, and an output terminal for outputting a signal into a next stage; and
a notch circuit connected to the first and second input terminals of the output buffer circuit and configured to have an impedance characteristic in which impedance is abruptly reduced to provide a valley point at an undesired signal frequency, in order to remove a component with the undesired signal frequency from the difference frequency signal, the notch circuit comprising first and second LC series resonance circuits connected to the first and second output terminals of the multiplying circuit, respectively, and having a resonance frequency in agreement with the undesired signal frequency.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.